How and where can I start learning AI if I have considerable knowledge of Java & C++?
If you are coming from Java, it would make a lot of sense to play around with deeplearning4J at first. From there I would start learning python as this is the primary language used today in ML. 

Lectures are a great way to get your feet wet in understanding and applying ML concepts. 

My favorites are:

Stanford ML(Andrew NG): https://www.youtube.com/watch?v=UzxYlbK2c7E (covers classic algorithms in depth)

MIT AI(Patrick Winston): https://www.youtube.com/watch?v=TjZBTDzGeGg&amp;t=2s (great starter course that covers basic concepts beautifully)

Deepmind RL(David Silver): https://www.youtube.com/watch?v=2pWv7GOvuf0&amp;t=7s (Best RL course available) 

Beyond that, start implementing these concepts. There are hundreds of tutorials on computer vision, natural language processing, and reinforcement learning.